When priming a positive displacement pump, which valve must always be closed?

When priming a positive displacement pump, it is essential to keep the discharge valve closed. The discharge valve must be closed to create a sufficient pressure difference that enables the pump to draw liquid into the suction line and fill the pump chamber. If the discharge valve is open during priming, the fluid can exit the pump freely without allowing it to build the necessary suction, making it difficult, if not impossible, for the pump to establish prime and function correctly.

In a positive displacement pump, the mechanism intrinsically relies on moving a specific volume of liquid with each cycle. Therefore, the closed discharge valve ensures that the liquid being drawn in from the inlet has nowhere to go. This allows the pump to become filled with liquid, effectively forming a seal, which is critical for its operation.

Thus, the correct approach when priming a positive displacement pump involves closing the discharge valve while ensuring the pump can adequately draw in fluid from the inlet. This is why keeping the discharge valve closed during priming is crucial for the pump's efficiency and performance.